Some organic solvents have high solubility in water. When such water-soluble organic solvents are collected after being used so as to be reused, liquid mixtures containing the organic solvents and water are often collected, so that it is required to separate the organic solvents targeted for reuse from the liquid mixtures and then purify the organic solvents. The collected liquid mixtures are likely to contain impurities, such as ionic materials and fine particles, other than the organic solvents and water. Depending on the usage manner or the collecting manner of the organic solvents, the liquid mixtures might contain dissolved gases, such as dissolved oxygen and dissolved carbon dioxide.
NMP that is one of organic solvents having high solubility in water is widely used, for example, as a dispersion medium for slurry in which particles such as electrode active materials are dispersed when the slurry is applied onto electrode collectors and dried to form electrodes in a manufacturing process of lithium ion secondary batteries. NMP is collected while the slurry is dried, and the collected NMP can be reused after being purified. In the collection of NMP, vaporized NMP is collected by a water scrubber, for example. Hence, NMP is collected as a liquid mixture in which NMP and water are mixed. At this time, NMP concentration in this collected liquid mixture is approximately 70 to 90 mass %. Since the water scrubber is used, oxygen and carbon dioxide derived from the atmosphere are dissolved in the liquid mixture.
As a conventional method of separating and recovering an organic solvent from a liquid mixture containing the organic solvent and water, a distillation method has been known. More particularly, a vacuum distillation method of reducing pressure of the liquid mixture to distil this liquid mixture has been often used. However, there are some problems in the distillation method and the vacuum distillation method that these methods require a large amount of energy, and require a large-scale distillation facility in order to purify the organic solvent to a desired purity level. To cope with this, there has been known a pervaporation (PV) method as a separation method requiring no large-scale facility and excellent in energy saving performance.
In the pervaporation method, a separation membrane having high affinity with a component targeted for separation processing, such as water, is used. A liquid mixture containing this target component, such as a liquid mixture containing an organic solvent and water is brought to flow toward a supply side of the separation membrane, and pressure is reduced or an inert gas is brought to flow on a permeation side of the separation membrane, thereby carrying out separation utilizing differences in permeation rate among respective components through the separation membrane. A separation membrane used in the pervaporation method is also referred to as a pervaporation membrane. As a separation membrane through which water is brought to pass, a zeolite membrane is used, for example. If only water component moves toward the permeation side through the separation membrane, the organic solvent remains on the supply side of the separation membrane, thereby recovering the organic solvent. If separation between water and the organic solvent is carried out with the pervaporation method, heating is required for efficient separation. As a removal method of ionic impurities contained in organic solvents, there has been known a method using an ion exchange resin, for example.
Patent Literature 1 discloses, as an NMP separation system to separate NMP from a liquid mixture of NMP and water, a system in which a pervaporation apparatus is used and an ion exchanger is provided subsequent to the pervaporation apparatus.
FIG. 1 illustrates an example of a configuration of an organic solvent purification system in background art, equipped with a pervaporation apparatus and an ion exchanger disposed subsequent to this pervaporation apparatus. Herein, the system shown in FIG. 1 will be described, assuming that the organic solvent is NMP, for example. A liquid mixture containing NMP and water at ordinary temperature is heated up to approximately 120° C. by heater 12, and is then supplied to pervaporation apparatus 13. Steam is used as a heat source of heater 12. Inside pervaporation apparatus 13, there is provided pervaporation membrane 14 made of zeolite, for example. Water in the liquid mixture passes through pervaporation membrane 14, and thereafter, is cooled to be condensed by condenser 16, and is then discharged. Meanwhile, NMP does not pass through pervaporation membrane 14, and thus NMP is directly discharged in its liquid state from a concentration side of pervaporation apparatus 13. NMP discharged from pervaporation apparatus 13 is cooled by cooler 15. NMP at ordinary temperature obtained in this manner is then supplied to ion exchanger 17 so as to remove ionic impurities therefrom. Furthermore, fine particles are removed from this NMP by microfiltration (MF) membrane 18, and then NMP is stored as purified NMP in a tank or the like, or is sent to a process where this NMP is used.
In the organic solvent purification system shown in FIG. 1, there is a problem that, if an ion exchange resin inside ion exchanger 17 is broken, impurities such as sodium and silicon derived from a separation membrane and a filtration membrane located in the system might remain in the purified organic solvent such as NMP. Ion exchanger 17 is provided subsequent to pervaporation apparatus 13. Since ion exchanger 17 is required to remove ions from NMP that is a non-aqueous solvent, there is a problem that an ion exchange efficiency is small and thus a great labor is also required for replacement of the ion exchange resin.
After the organic solvent is separated from water by the pervaporation apparatus, as a method of further purifying this organic solvent, there has been known a method in which an evaporator is provided subsequent to the pervaporation apparatus so as to distil the organic solvent by this evaporator. This method is used for alcohol purification or the like. FIG. 2 illustrates an example of a configuration of an organic solvent purification system of background art in which the pervaporation apparatus and the evaporator are combined. In this system, ion exchanger 17 and microfiltration membrane 18 are omitted from the system shown in FIG. 1, and evaporator 20 heated by steam is provided between a concentration side of pervaporation apparatus 13 and cooler 15, instead. The organic solvent obtained from the concentration side of pervaporation apparatus 13 is distilled and purified in evaporator 20, and is condensed and cooled in cooler 15. Thereafter, the purified organic solvent is reserved in a tank or the like, or is sent to a process where the organic solvent is used. The ionic impurities, the fine particles and the like having been contained in the organic solvent are left in evaporator 20.